It may not look as beautiful as a Gaussian blur, but I think that
a box blur can't get any faster than this. The specialty about
this algorithm is that its speed is almost independent of the used radius.
This works, because the convolution kernel is not recalculated on each pixel:
scanning through the image the algorithm just adds one new pixel to the kernel
and at the same time drops one old pixel from it.

The Y-position of your mouse controls the radius of the blur in this example.